The present invention deals with one of the vital problems of photothermal recording and storage of digital information, which is effected by means of optical storage devices capable of erasing information and recording it again. The existing methods of reversible recording of data, which make use of reversible magnetooptical and structural transformations in the recording media based on chalcogenide resins are extremely difficult to realize.
Known in the art is a method of photothermal information recording, reading and erasing, based on hydrodynamic effect in heated thin films of liquid (A. L. Kartuzhansky, Neserebrianye Protsessy Tonkikh Plenok Zhidkosti, 1984, Khimiya Publ., Leningrad, pp. 231-232), which consists in that changes in the geometry of the thinfilm surface, corresponding to information being recorded and dictated by the temperature relations of the surface energy in this film, are obtained by irradiating the film which absorbs this radiation.
This prior art method is deficient in that the storage period is too short and the sensitivity and resolution are too low, which is inherent in the liquid film characterized by thermal quasi-equlibrium of hydrodynamic relaxation.
Also known in the art is a method of photothermal information recording, reading and erasing (A.L. Kartuzhansky, Neserebrianye Fotograficheskie Protsessy, 1984, Khimiya Publ., Leningrad, pp. 44-49), comprising the steps of producing a potential pattern in the recording medium film by preliminary uniform charging of this film surface by a corona discharge, projecting an image to be recorded onto the film in the spectral range corresponding to the photoconductivity of the film material, developing the latent pattern by simultaneously softening the entire film by heating, and fixing the pattern, e.g. by cooling the film surface. The pattern recorded by this method is erased by heating and softening the recording medium.
The prior art method of photothermal information recording, reading and erasing can be used for multiple information rewriting. It is highly sensitive and offers a long period of storage. But this method is deficient in that the level of noise is too high because of the inherent deformations of photoplastic materials. Moreover, duration of one recording-erasing cycle is too long due to the contributing factors, such as the time required for charging the surface of the film, low mobility of current carriers in the photoplastic material, and the period of thermal relaxation of the recording medium. The resolution of the method is not high enough for the information recording and reading to be performed with a density provided by optical systems. In addition, the transmission band is too narrow. These deficiencies are inherent in the techniques of producing a latent pattern and developing this pattern over the entire surface of the thin film. To realize this prior art method, it is necessary, at first, to delineate a latent pattern which is a varying density of the surface charge throughout the film surface. Such variations in the surface charge density are dictated by the brightness of the pattern projected onto the film surface. This imaging technique is time-consuming and is the one of the sources of noises. The latent pattern is developed by simultaneous softening of the entire film and this brings out, as a surface relief pattern, not only the latent image but also interferences. In addition, heating and subsequent cooling of the entire recording medium requires too much time. To summarize, this prior art method cannot provide optimal conditions for film deformation during photothermal recording and erasing of an information unit. It cannot provide photothermal recording and erasing of information so that the information unit is localized to a size limit ensured by the resolution of optical systems.
The closest prior art is a method for photothermal information recording, reading and erasing Vtoraya vsesojuznaya nauchno-tekhnicheskaya konferentsiya "Problemy razvitiya Radiotekhniki", 1985, Tsentralnoye i gruzinskoye respublikanskoe pravleniye nauchno-tekhnicheskogo obschestva radiotekhniki elektroniki i svyazi imeni A. S. Popova, Moscow, pp. 62-63), in which information is recorded by exposing a recording medium on a substrate, which is capable of spreading over this substrate when melted, to a modulated electromagnetic beam localized to a spot size of the order of one information unit, using this electromagnetic beam to melt and change the pattern of the recording medium, and then fixing thus obtained localized information units in the form of two-level pits by cooling the recording medium. This is the end of the photothermal process of information recording. Readout of pits is performed by a focused electromagnetic beam. Recorded information can be erased by heating the entire surface of the recording medium.
This prior art method offers the advantage of discrete photothermal information recording and erasing within short periods of time dictated by the duration of electromagnetic pulses with the density offered by optical systems and a high signal-to-noise ratio. But this method is deficient in that the reliability of information processing is not high and the number of rewrite cycles is low. The reason is that the recording medium has to satisfy several conflicting requirements involved in the processes of information recording and erasing. In fact, since the surface relief pattern of the recording medium is changed due to the gradient of forces of the surface tension and hydrodynamic stress, it should be heated to a temperature substantially exceeding its melting point in order to improve the viscoelastic properties of the recording medium and raise the probability of pit centers to appear. But this temperature results in fast destruction of the material of the recording medium and drastically reduces the number of rewrite cycles which could be realized. High-power continuous lasers are to be used to heat the recording medium for recording and this results in a considerable increase in weight and dimensions of devices for photothermal recording, reading and erasing of information. It seems that the remedy is to use thin-film recording media in order to bring down the required beam power. Such thin films possess low surface energy since they are made of materials which do not wet the substrate when liquid. But this runs contrary to the requirements set to such films by conditions for erasing information whereby the recording medium is to be stable to external exposure for reliable information storage.
The prior art method makes use of a recording medium whose surface energy cannot be controlled during information recording, reading and erasing. Requirements set to the surface energy the recording medium must posses during recording and erasing are conflicting. To optimize conditions for information recording, reading and erasing, it is necessary to reduce the surface energy of the recording medium film during recording and to increase this energy during erasing of information.
The prior art method offers no way to control the surface energy of the recording medium film during information recording, reading and erasing in order to provide optimal conditions for these processes.